Three-Dimensional Hydrodynamic Simulations Indicate Instantaneous Pathways for Fish Passage

Allowing fish to migrate freely is a primary goal for conserving migratory fish species and more generally for entire fish communities. In recent years, substantial effort has been invested in developing, constructing, and improving fish passage facilities to help migratory species navigate barriers in river ecosystems. With continuously growing capacities of computers, numerical simulations have become standard tools to hydraulically optimize such fishways. However, the information obtained by three-dimensional (3d) simulations is merely leveraged and rarely subject to thorough analysis. This study features a 3d hydrodynamic numerical model of a vertical slot fishway (VSF) to explore the relevance of the 3d-velocity information, which is not available from depth (2d) or cross section-averaged (1d) simulations for ecohydraulic assessments. A multiphase solver within the OpenFOAM software is used to investigate the advantages of 3d information compared to depth-averaged models. Specifically, vertical velocity profiles provide insights into near-bottom and near-wall regions where fish could pass, even when a depth-averaged model would suggest that the mean velocity is too high for fish passage. In addition, hydrodynamic variations in turbulent kinetic energy patterns show that even in steady flow conditions, there are high temporal variations of flow velocity patterns, which provide instantaneously changing opportunities for fish to migrate through slots. Also, resting zones were identified within pools of the VSF, which represent valuable features for a fishway that might otherwise cause exhaustion of fish after passing a couple of pools. Thus, design improvements with 3d simulations may also embrace the optimization of such resting zones.